This invention deals with improving the sensitivity of defect inspection on patterned surfaces where the patterned structures are not fabricated as intended. The inspection of these surfaces may involve directing light onto such surfaces, collecting light from the surface and processing the collected light to determine whether defects are present. An example of this can be in semiconductor wafer manufacturing where thin-film layers are processed with lithography creating patterns in the surface that are subsequently etched and processed to create semiconductor devices. In the lithography process, a layer of photoresist is deposited on the surface and the photoresist is illuminated with a pattern that is developed and processed to establish a pattern that will be etched into the surface to create a layer for the semiconductor device. Defects within photoresist layers are an example of low-signal producing defects where some amplification of detection signals may be required to adequately identify defects. This need may be higher when considering the extreme ultraviolet (EUV) lithography requirements for detecting printed defects from the mask. Prior methods for accounting for low level signals included optimization of the inspection parameters of light, spectral band, aperture mode and tool speed/pixel to identify an optimal inspection recipe.
However, such optimizations may be insufficient for detecting signal levels on print-check wafers (sometimes called “flop-down wafers”) resulting from ever-smaller defects in future design rules (DR) (e.g. a 15 nm DR having allowed excursions of 1/10 of the line width or 1.5 nm) established from EUV mask inspection requirements.